In scientific sounding terms, it's what happens when sexual excitement has been building - an orgasm is the highest point of excitement, and is a 'release.'
As to what one actually feels like, that's different for all people. For females, it's particularly unique.
A female might have one big orgasm that feels like fireworks, or one that ripples through her body. She might have a series of smaller orgasms as well - called multiple orgasms.
The best way to figure out what one is, is to explore yourself. Spend some time alone and working out what feels good to you. An orgasm is hard to miss and you will definitely know when you have had one - best of luck.